The material and structure of conventional varistor made from zinc oxide are composed of oxides, such as B, Bi, Ba, Si, Sr, Pb, Pr, Co, Mn, Sb or Cr or their mixtures. The oxide, such as bismuth oxide, forms a crystal boundary between the particles of zinc oxide and the density of this material shall be maintained close to the theoretical density structure, usually more than 90% of the theoretical density. The material is a commercialized product, but its capacitance value is too high. The crystal layer is similar to capacitors in the performance of electrical property and brings a high capacitance value to the varistor made from it so that these varistors are not suitable for RF circuits. This is the major disadvantage of this kind of varistor. The aforesaid material for the transient voltage suppressors in said invention has a loosely stacked structure and, therefore, can provide a lower capacitance value and leakage current even using the same material ingredients and component designs. It is, therefore, suitable for the use in RF circuits and antennas.
The breakdown voltage of varistor made from zinc oxide is related to the number of crystal boundary between the two electrodes. There is a crystal boundary between the crystals of zinc oxide. This is a non-linear resistance interface between zinc oxide crystal. Assuming that N stands for the semiconductor made from zinc oxide and P stands for the crystal boundary, the structure of varistor from one end of the electrode to the other end may be described as:
E-P-N-P-N- . . . -N-P-N-P-E
Wherein E stands for the electrodes of the conductor. A breakdown voltage Vb1 exists for each P-N interface. Assuming that there are several P-N interfaces between the electrodes, the breakdown voltage Vb may be described as the sum of Vb1.
The invention that is still under patent application is a kind of loosely stacked material with non-linear resistance interfaces. Taking the powder with non-linear resistance interface composed of oxides, such as B, Bi, Ba, Si, Sr, Pb, Pr, Co. Mn, Sb or Cr, or their mixture sintered with zinc oxide powder as an example. There are some powders with non-linear resistance interface between the two electrodes of the component. Assuming that P stands for composed of oxides, such as B, Bi, Ba, Si, Sr, Pb, Pr, Co., Mn, Sb or Cr, or their mixture and N stands for zinc oxide powder. The structure of the invented transient voltage protection components from one end of the electrode to the other end may be described as:
E-P-N-P-S-P-N-P- . . . -P-N-P-S-P-N-P-E
Wherein P-N-P stands for a crystal, while S stands for the space layer between crystals. S can be an insulator of air or glass and the breakdown voltage therein is represented by Vs. S may also not exist due to the contact of crystals. Breakdown voltage Vb2 exists for each P-N-P. Because there are a number of crystals between the electrodes, the breakdown voltage of the components may be described as the sum of Vb2 plus the sum of Vs.
The material for voltage suppressors that was published in U.S. Pat. No. 4,726,991 was composed of a conductor or semiconductor powder coated with an insulating layer with a thickness less than several hundreds of angstroms. This structure, however, has some disadvantages in practical use. First, the thickness of the insulating layer is within several hundreds of angstroms and makes the material very difficult to be produced in the manufacturing process. If the coated insulating layer is too thin, the component will short-circuit easily. If the coated insulating layer is slightly thicker, the breakdown voltage will be increased. This is the fault caused by putting insulating layer over the surface of conductor or semiconductor powder.
Another coating material has been published in U.S. Pat. No. 5,294,374 the structure of which is a mixture of a conductive powder coated with an insulating layer and a semiconductor powder without any insulating layer. The thickness of the coated insulating layer is between 70 angstroms and 1 micron. The coating layer can be made from semiconductors. Basically, the coating layer is made from insulating or semiconductor material that can prevent current flow and create high resistance. The thickness of the coating layer directly determines the breakdown voltage of components, therefore, it is important to maintain an even thickness.
All kinds of conductor, semiconductor or insulator powders are uniformly mixed in the variable resistance material containing binder which have been published in US Patent articles with U.S. Pat. Nos. 3,685,026, 3,685,028, 4,977,357, 5,068,634, 5,260,848, 5,294,374, 5,393,596 and 5,807,509 respectively. These powders do not have the characteristics of non-linear resistance in their particle and the action of the breakdown voltage was derived from the composition of these powders, which is different from said invention. The material structure of said invention, therefore, has the characteristics of novelty and practicability.
The purpose of said invention is to provide a material for transient voltage suppressors. The material of the invented transient voltage protection material is a mixture of at least two kinds of powders including a powder material with non-linear resistance interfaces and a conductive powder material. The loosely stacked structures of these materials is formed by mixing and sintering these powders uniformly to reduce the total number of non-linear interfaces between the electrodes and, as a result, the breakdown voltage of the components is decreased.
The transient voltage suppressors made from the invented material are applicable to many kinds of component structures. FIG. 1 shows the practical example of the feasible structure of the invented transient voltage suppressors in which the insulating plate 20 is used as the matrix. The conducting electrodes 22 and 24 are formed t on the plate with the positions of the two electrodes on the same plane and a gap 28 between the two electrodes. A kind of component structure can be produced by filling the invented powder material 26 in the gap and heating it appropriately to make the powder material stack up to be a loose structure.
FIG. 2 shows another practical example of the feasible structure of the invented transient voltage suppressors in which the invented powder material is used as the matrix. After the invented powder material is sintered to be blocks, the electrode 34 is formed on the top of the material, while the other electrode 32 is formed on the bottom of the material. This sandwich component structure forms another structure of the transient voltage suppressors.
As for the detailed structure, application principles, functions and performance of said invention, please refer to the following figures for further understanding.